1. Field of the Invention
This invention relates to a method of manufacturing a spherical body comprising a core and at least one surface layer enclosing the core. The invention relates also to spherical bodies manufactured by this method, and especially two-piece and multi-piece golf balls.
2. Prior Art
Two-piece golf balls, which have become very popular in recent years, are manufactured by placing a special rubber core as the nest within a two-part mold having upper and lower halves, each with a hemispherical concave inner face, holding the core at the center with pins provided for retraction near the bottom and top of the inner face of the mold, and injection molding a thermoplastic resin such as ionomer resin in the gap between the core and the inner face of the mold.
The manufacture of multi-piece golf balls having two or more surface layers requires the same number of surface layer-forming molds, such as a mold for the inner surface layer and another for the outer surface layer, as there are surface layers. The injection molding process described above is carried out a plurality of times with different molding materials.
However, efforts to form very thin surface layers having a thickness of 1.5 mm or less in the gap between the core serving as the nest and the inner face of the mold using a prior-art injection molding process have met with difficulties. The thermoplastic resin material which has been forced through a runner from the injection molding machine cools and prematurely cures as it is spreads throughout the entire region of the gap, making it likely that unduly thin-walled sections and unfilled areas will arise.
Moreover, the molding of multi-piece balls, which have a multilayer construction, requires that a plurality of molds of differing inner face diameters be furnished and that injection molding be carried out a plurality of times, once for each surface layer. This in turn increases the equipment and labor involved in producing the balls.